Semiconductor products are generally produced via batch processing steps that use gases to deposit or selectively etch semiconductor layers on substrates within a vacuum chamber. Most of the chemical by-products and unused reagents from these deposition and etch processes are exhausted from the chamber by a vacuum pump. However, some residue unavoidably deposits on the chamber walls and must be removed periodically in order to maintain product quality. Usually this residue is removed with gas mixtures containing some fluorine-containing cleaning gas, such as NF3, SF6, C2F6, or CF4, which is usually diluted with argon or helium.
Unfortunately, SF6, NF3, C2F6, and CF4 have very high global warming potentials, i.e. respectively about 23,900, 10,090, 9,200, 6,500 times CO2 on a weight average basis over a 100 year time-frame, respectively. While some fluorine containing cleaning gases have much lower global warming potentials, F2 and ClF3 for example, these cleaning gases are very toxic, highly reactive, and difficult to handle safely. These problems are exacerbated by the more recent trend to use semiconductor production techniques for the production of larger and larger flat panel displays that require a significant increase in the quantity of chamber cleaning gas. In particular, there is a significant increase in the associated environmental and safety issues. Moreover, because flat panel displays have much lower product prices per unit area than computer central processing or memory module units, non-productive cleaning time and the cleaning gas cost represent an increasing share of the total flat panel display cost. Therefore, there is a need in the art to ameliorate environmental concerns while maintaining safety and process efficiency.
NF3 is the most common chamber cleaning gas and is typically produced by the reaction of fluorine with a NH4F(HF)x salt, such as by the following reaction:3F2+NH4F(HF)x→NF3+(4+x)HF.The reaction may be carried out in an electrolytic cell (as shown in U.S. Pat. No. 3,235,474) or in a separate reactor (as shown in U.S. Pat. No. 4,091,081). Alternatively, NF3 production from urea and fluorine has been proposed (as shown in U.S. Pat. No. 6,821,496) using the following key step:2CO(NH2)2+3F2→NF3+NH2CONHCONH2+3HF.
All these ammonia-based NF3 production processes use half of the fluorine feed to produce NF3 and the other half to produce HF. Therefore, the direct use of fluorine as a chamber cleaning gas would be much more efficient than NF3.
Although F2 is a more efficient and theoretically lower cost chamber cleaning gas than NF3, elemental fluorine has generally not been used because of cylinder shipping and handling safety concerns. On-site fluorine production, via electrolysis of hydrogen fluoride (as described in US Published patent application 2003/0098038), has been suggested as an approach to eliminate the fluorine cylinder handling problems, as well as to decrease global warming emissions, and increase the fluorine use efficiency. However, on-site fluorine production faces two significant challenges.
First, the quantity of the fluorine product that can be safely stored is severely limited by fluorine's high reactivity and toxicity. As a result, significant fluorine plant excess capacity is required to meet the highly variable cleaning gas flow rate requirements of a typical semiconductor production facility. In addition, the fluorine plant must be designed to minimize the probability that a fluorine plant outage and a disruption in semiconductor production. The risk of an outage and the very high opportunity cost for semiconductor plant outages economically justifies a separate back-up cleaning gas supply capability, usually NF3. Therefore, the commercial need for a highly reliable chamber cleaning gas feed system and the highly toxic and reactive nature of fluorine generally requires an oversized and more expensive fluorine production facility as well as a back-up NF3 supply system. In such a case, the theoretical cost savings can not be realized.
Second, the hydrogen fluoride feed necessary for fluorine production is also highly toxic and volatile. Therefore, the large hydrogen fluorine feed inventories required, especially for flat panel display plants, pose a significant health risk that must be mitigated. For this reason, large-scale fluorine production facilities are usually located in relatively sparsely populated areas with a large buffer land area around the production facility. However, large-area display production facilities are often located in areas with high population densities and land prices. Therefore, there remains a need for a flexible fluorine and nitrogen trifluoride production and supply capability that avoids large inventories of toxic and volatile feeds and products.